The present invention relates to electrical power distribution systems and more particularly to apparatus for protecting electrical power distribution lines and equipment.
Protection of power lines and power line distribution equipment is essential in order to provide constant service to all utility customers. Protective relaying is one form of power line protection used by utilities to provide a safe, reliable system of electrical power distribution. To prevent permanent costly damage from occurring to the distribution networks of the utilities, the protective relaying system must be able to clear short-term faults and isolate the protected zone during phase-to-ground, phase-to-phase and three phase faults quickly and reliably. One method of providing this protection is pilot wire relay protection.
Pilot wire relay protection typically utilizes two electro-mechanical relays, one at each end of a protected zone, and a communication channel connecting these relays. Typically, the communication channel is a dedicated telephone line provided by the telephone company which is leased or purchased by the power company. Heretofore, these dedicated paths provided a hardwire current path between the relays. This proved to be very effective in performing pilot wire relay type protection of electrical power distribution lines because the hard wired communication channels allowed for simple current flow across the pilot wire which would close the relays and trip the associated circuit breakers thereby isolating the fault in the electrical power distribution system.
However, with the perfection of non-metallic communication channels, the telephone companies have been upgrading their systems by replacing hard wired telephone communication lines with non-metallic communication channels. Such upgrading presents problems for the power companies which utilize pilot wire relay protection. With the installation of a non-metallic communication channel, there is no longer a wire communication channel over which the current can flow. In addition, the non-metallic communication channels being used by telephone companies, such as The Bell Telephone Series 3002 voice-grade communication channels, have a limited bandwidth in the audio band of approximately 300 to 3000 Hz. The signals which typical pilot wire relays produce occur at 60 Hz. Since such signals are below the lower limit of the bandwidth of such non-metallic communication channels, they will not be transmitted across a communication channel of this type.
It is therefore an object of the present invention to enable pilot wire relays to communicate across voice-grade non-metallic communication channels.
It is another object of the present invention to enable pilot wire relays to communicate over a voice grade lines comprised of either loaded cable pairs, coaxial cable, microwave/cellular systems as well as fibre optic communication channels.
It is yet another object of the present invention to enable pilot wire relays to communicate over such voice grade lines without having to replace or internally modify the existing pilot wire relays.
It is a further object of the present invention to provide means of communicating 60 Hz pilot wire relay signals using audio tones which effectively preserve phase comparison principles and techniques.
It is also an object of the present invention to effectively use audio tones to communicate 60 Hz pilot signals which enable direct transfer trip signals to be transmitted down the communication channel and which provide means for a dual tone direct transfer trip apparatus to accompany the phase comparison audio tone scheme.
It is still another object of the present invention to enable pilot wire relays to communicate over non-metallic communication systems while still preserving true phase comparison signals of both the local and remote sites to be compared.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.